The invention relates to a process for the preparation of crystalline iron silicates with improved catalytic properties.
Mixtures of carbon monoxide and hydrogen can be converted into aromatic hydrocarbon mixtures using a mixture of two catalysts of which one has the capability of catalyzing the conversion of an H.sub.2 /CO mixture into acyclic oxygen-containing hydrocarbons and the other is a crystalline iron or aluminum silicate which has the capability of catalyzing the conversion of acyclic oxygen-containing hydrocarbons into aromatic hydrocarbons. The said crystalline iron and aluminum silicates are characterized in having the following properties:
(a) thermally stable up to a temperature above 600.degree. C., PA0 (b) an X-ray powder diffraction pattern showing, inter alia, the reflections given in Table A, PA0 wherein the letters used have the following meanings: VS=very strong; S=strong; M=moderate; W=weak; .theta.=angle according to Bragg. PA0 (c) in the formula which gives the composition of the silicate, expressed in moles of the oxides, and in which, in addition to oxides of hydrogen, alkali metal and silicon, there is present an oxide of a trivalent metal A selected from Al and Fe, the A.sub.2 O.sub.3 /SiO.sub.2 molar ratio (for the sake of brevity further designated m in this patent application) is less than 0.1. PA0 M.sub.2 O:(R.sub.4 N).sub.2 O=0.05-5 PA0 (R.sub.4 N).sub.2 O:SiO.sub.2 =0.05-1 PA0 SiO.sub.2 :A.sub.2 O.sub.3 &gt;10, and PA0 H.sub.2 O:SiO.sub.2 =5-50. PA0 M.sub.2 O:(R.sub.4 N).sub.2 O=0.4-1.0, PA0 [M.sub.2 O+(R.sub.4 N).sub.2 O]:SiO.sub.2 =0.24-0.40, PA0 SiO.sub.2 :Fe.sub.2 O.sub.3 &gt;10, and PA0 H.sub.2 O:SiO.sub.2 =5-50,
TABLE A ______________________________________ Radiation: Cu--K.alpha. Wavelength 0.15418 nm 2 .theta. relative intensity ______________________________________ 7.8-8.2 S 8.7-9.1 M 11.8-12.1 W 12.4-12.7 W 14.6-14.9 W 15.4-15.7 W 15.8-16.1 W 17.6-17.9 W 19.2-19.5 W 20.2-20.6 W 20.7-21.1 W 23.1-23.4 VS 23.8-24.1 VS 24.2-24.8 S 29.7-30.1 M ______________________________________
The crystalline silicates which are used in the catalyst mixtures may be prepared starting from an aqueous mixture which contains the following compounds: one or more compounds of an alkali metal (M), one or more quaternary alkylammonium compounds (R.sub.4 NX), one or more silicon compounds and one or more iron or aluminum compounds. The preparation is effected by maintaining the mixture at elevated temperature until the crystalline silicate has been formed, separating it from the mother liquor and calcining it. In the aqueous mixture from which the silicates are prepared the various compounds should be present in the following ratio, expressed in moles of the oxides:
In an investigation by the Applicant concerning the use of the above-mentioned catalyst mixtures, in which an iron silicate has been incorporated as the crystalline silicate, for the preparation of aromatic hydrocarbon mixtures starting from H.sub.2 /CO mixtures with an H.sub.2 /CO molar ratio lower than 1.0, the activity of these catalyst mixtures was often found to be unsatisfactory.
Further investigation by the Applicant concerning this subject showed that the activity of these catalyst mixtures is greatly determined by the [M.sub.2 O+(R.sub.4 N).sub.2 O]/SiO.sub.2 and M.sub.2 O/(R.sub.4 N).sub.2 O molar ratios used in the aqueous mixture from which the crystalline iron silicate component of the catalyst mixture is prepared. It has been found that catalyst mixtures with optimum activity for the conversion of H.sub.2 /CO molar ratio below 1.0 into aromatic hydrocarbon mixtures can be obtained by using in the catalyst mixtures a crystalline iron silicate component which has been prepared from an aqueous mixture in which the [M.sub.2 O+(R.sub.4 N).sub.2 O]/SiO.sub.2 molar ratio is 0.24-0.40 and the M.sub.2 O/(R.sub.4 N).sub.2 O molar ratio 0.4-1.0. The preparation of crystalline iron silicates having the properties mentioned under (a)-(c) from an aqueous mixture in which the amounts of quaternary alkylammonium compounds, alkali metal compounds and silicon compounds have been so chosen that the above-mentioned requirements concerning the molar ratios are satisfied, is novel.